1. Field of the Invention
The present invention relates to an aluminum nitride powder.
2. Description of the Related Art
Since aluminum nitride is excellent in electrical insulation and has a high thermal conductivity, it is expected as a high thermal conductive material which is used as, for example, a raw material of a sintered body or a filler to be added to a resin and so on.
In particular, in the electronics field, with the increase of an integration degree of IC and an output of a power transistor, it is desired to effectively dissipate heat generated in a semiconductor device. Then, the application of an aluminum nitride material having a high thermal conductivity is highly expected.
A conventional aluminum nitride powder consists of particles grain boundaries of which are not clearly observed in a SEM (scanning electron microscope) photograph.
When such aluminum nitride powder is used in the production of a sintered body, the small particle powder is easily sintered, but the smaller particles are more greatly influenced by the van der Waals' force and strongly agglomerated, so that the powder becomes bulky and its handling is difficult. In addition, a density of a molded article before sintering decreases, and then the shrinkage after sintering increases. When a powder of large particles is used, its handling is easy and a density of the molded article increases, but its sintering is difficult.
When the aluminum nitride is added to a resin and used as a filler therein, since the conventional aluminum nitride powder has smooth particle surfaces, adhesion between the particles and the resin is insufficient. On the other hand, when the particle surfaces have unevenness, since they have voids, the resin hardly reaches their insides deeply, whereby some defects are caused. If the adhesion between the resin and the particles is insufficient, moisture absorbed from an air induces peeling off of an adhered interface, or moisture penetrates in a space formed by such peeling off so that an amount of absorbed water increases and then the aluminum nitride particles may be deteriorated.
With the aluminum nitride powder which can be used as a filler in a resin, many investigations have been made.
For example, Japanese Patent KOKAI Publication No. 23206/1991 discloses an aluminum nitride powder with a uniform single particle size which comprises particles having an average single particle size of at least 3 .mu.m each of which has a rounded shape and contains substantially no spinel aluminum oxynitride. This Publication includes a photograph of the aluminum nitride powder in FIG. 1, but the particle surfaces are smooth. When the particle surfaces are smooth, an anchoring effect due to unevenness of the particle surfaces cannot be expected, and therefore, the adhesion between the matrix resin and the particles is insufficient.
Japanese Patent KOKAI Publication No. 74705/1992 discloses spherical aluminum nitride particles having an average particle size of 7 to 300 .mu.m and a ratio of a longer axis to a shorter axis of 1.5 or less. This Publication includes a photograph of the aluminum nitride powder in FIG. 1. The powder comprises particles having deep holes. When the particles have the deep holes, the resin does not penetrate deeply into the holes, whereby some defects are caused.